Although evidences of heterogeneity have been extensively documented for a variety of enzymes, in the case of catalase a diversity of opinion still remains as to the extent and nature of its multiplicity. The heteromorphic quality of catalase has been unequivocally established (five components) in maize, protozoan Tetrahymena pyriformis, and in tissues of mice and rats. The property of enzyme heterogeneity usually transcends species barriers, but in the case of catalase it may be unique in that heteromorphs may be restricted to a few mammalian species. In the human, convincing data in support of catalase heteromorphs are still lacking. The technique of overlay immunoisoelectric focusing (OIIF), recently developed in our laboratory, has instilled some fresh insights with reference to the question of multiplicity of catalase in human erythrocytes. Five intensely reactive catalase bands (and a few very faint bands with minute activity) were repeatedly shown in lysed erythrocyte samples. The determination of isoelectric point, pI, for each of the intense bands was highly reproducible. The characteristic pI values of human erythrocytes were regularly observed in erythrocytes from individuals of several diverse ethnic backgrounds of either sex. This method therefore will be used to further characterize catalase heteromorphs of human tissues by attempting the separation of the actively reactive components into their fundamental units. Conversely, the re-combination of the proposed subunits will be attempted. A similar procedure for such analyses of tissues in inbred mice, (AxC57B16)F1 and S1/S1d, will be established. The evidence for the existence of possible temporal relationships between the appearance of different heteromorphic components and the stages of differentiation in erythroid and myeloid cells will be sought.